Prenatal care involves, among other things, measuring fetal length and weight to determine if the pregnancy is going well. Most doctors rely on growth charts that represent averages to determine if a fetus is developing normally. But in reality, most fetuses grow at different rates. If an ultrasound test shows that your baby is smaller or larger than what is considered “normal”, this does not necessarily mean that your baby isn’t healthy. In fact, an article published in the American Journal of Obstetrics & Gynecology states that fetuses with a weight that is less than the 10th percentile are not necessarily growth-restricted; they may simply be smaller than their peers. Now, with that being said, here is what you need to know about fetal length and weight week by week.
How Fetal Length and Weight Are Measured
After the first trimester of pregnancy, doctors may perform ultrasound testing to determine the stage of pregnancy. When fetal measurements are compared to menstrual dating, predicting the due date becomes more accurate. This also helps doctors determine if your baby is of appropriate size for its gestational age. The baby is measured from head to bottom (rump). Doctors will also measure the size of the thigh bone, head circumference, and abdomen circumference and using a mathematical formula, they will determine fetal length and fetal weight. They will then compare these numbers to averages and see if the baby is within the 10th percentile or the 90th percentile. Babies that are too small may be growth restricted and further testing is needed to determine if an illness is causing this. And babies that are too large may indicate gestational diabetes and require a caesarean section.
Fetal Growth in the Second Trimester
The second trimester is an exciting time as this is when you’ll notice your baby’s first kicks and find out the sex of your baby. During week 14, your baby may be 3 ½ inches long and weigh 1 ½ ounces. The baby has developed arms and a defined neck, and the spleen is producing red blood cells. By the 27th week of pregnancy, the baby will be around 14 inches and weight almost 2 pounds. At this stage, your baby’s lungs and nervous system are maturing. If your baby weighs more than 2.7 pounds at this point, this may indicate that your baby is at the 90th percentile. However, keep in mind that these are just averages and medical experts now agree that most pregnancies fall outside of these categories. Studies are now working on developing individualized charts that could offer a better representation of what is normal and what not.
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Fetal Growth in the Third Trimester
During the third trimester, your baby will gain more weight and prepare for delivery. At the beginning of the third trimester, in 28th week of pregnancy, the baby’s eyes open for the first time. The baby could be at this time around 10 inches long and weigh 2 ¼ pounds. At the end of the trimester, just before birth and at 40th week of pregnancy, your baby should be between 18 and 20 inches long and weigh around 6 ½ pounds or even up to 8 pounds. Remember, most babies come in different sizes and this can depend on your own size and genetics. You may also deliver a week or two later and earlier than expected at this time which is also completely normal.
|Gestational age||Length (US)||Weight (US)||Length (cm)||Mass (g)|
|8 weeks||0.63 inch||0.04 ounce||1.6 cm||1 gram|
|9 weeks||0.90 inch||0.07 ounce||2.3 cm||2 grams|
|10 weeks||1.22 inch||0.14 ounce||3.1 cm||4 grams|
|11 weeks||1.61 inch||0.25 ounce||4.1 cm||7 grams|
|12 weeks||2.13 inches||0.49 ounce||5.4 cm||14 grams|
|13 weeks||2.91 inches||0.81 ounce||7.4 cm||23 grams|
|14 weeks||3.42 inches||1.52 ounce||8.7 cm||43 grams|
|15 weeks||3.98 inches||2.47 ounces||10.1 cm||70 grams|
|16 weeks||4.57 inches||3.53 ounces||11.6 cm||100 grams|
|17 weeks||5.12 inches||4.94 ounces||13 cm||140 grams|
|18 weeks||5.59 inches||6.70 ounces||14.2 cm||190 grams|
|19 weeks||6.02 inches||8.47 ounces||15.3 cm||240 grams|
|20 weeks||6.46 inches||10.58 ounces||16.4 cm||300 grams|
|20 weeks||10.08 inches||10.58 ounces||25.6 cm||300 grams|
|21 weeks||10.51 inches||12.70 ounces||26.7 cm||360 grams|
|22 weeks||10.94 inches||15.17 ounces||27.8 cm||430 grams|
|23 weeks||11.38 inches||1.10 pound||28.9 cm||501 grams|
|24 weeks||11.81 inches||1.32 pound||15.3 cm||600 grams|
|25 weeks||13.62 inches||1.46 pound||34.6 cm||660 grams|
|26 weeks||14.02 inches||1.68 pound||35.6 cm||760 grams|
|27 weeks||14.41 inches||1.93 pound||36.6 cm||875 grams|
|28 weeks||14.80 inches||2.22 pounds||37.6 cm||1005 grams|
|29 weeks||15.2 inches||2.54 pounds||38.6 cm||1153 grams|
|30 weeks||15.71 inches||2.91 pounds||39.9 cm||1319 grams|
|31 weeks||16.18 inches||3.31 pounds||41.1 cm||1502 grams|
|32 weeks||16.69 inches||3.75 pounds||42.4 cm||1702 grams|
|33 weeks||17.20 inches||4.23 pounds||43.7 cm||1918 grams|
|34 weeks||17.72 inches||4.73 pounds||45 cm||2146 grams|
|35 weeks||18.19 inches||5.25 pounds||46.2 cm||2383 grams|
|36 weeks||18.66 inches||5.78 pounds||47.4 cm||2622 grams|
|37 weeks||19.13 inches||6.30 pounds||48.6 cm||2859 grams|
|38 weeks||19.61 inches||6.80 pounds||49.8 cm||3083 grams|
|39 weeks||19.96 inches||7.25 pounds||50.7 cm||3288 grams|
|40 weeks||20.16 inches||7.63 pounds||51.2 cm||3462 grams|
|41 weeks||20.35 inches||7.93 pounds||51.7 cm||3597 grams|
|42 weeks||20.28 inches||8.12 pounds||51.5 cm||3685 grams|
What If My Baby Is Not In The Normal Percentile?
If your doctor says that you don’t have reason to worry, then there really is no reason to worry. Most fetuses fall outside the “normal” range. If the doctor sees that the fetus is far beyond what would be considered normal, then you will probably need to undergo additional checkups to determine what is causing this discrepancy. However, your own body size, weight, ethnicity, and lifestyle can all affect fetal size. Besides, there are even cases where a baby that has a weight above the 10th percentile did not denote the development was normal. This was especially true in cases during late gestation where the baby fell within the normal weight but actually experienced growth restriction.
Mothers-to-be often worry about their baby’s development. The huge amount of information they receive during their pregnancy can sound confusing and scary even. This is especially true when speaking of percentiles and averages. However, just like children develop at a different rate, so do fetuses. Unfortunately, doctors have to rely on number averages upon which they can take measurements. This might bring about a great deal of confusion, but nevertheless, is necessary. If you have any worries about the size of your growing baby, do talk to your doctor about your concerns and ask if they could explain your current situation in greater detail.